Answer:
![\boxed {\tt 3 \ hours}](https://img.qammunity.org/2021/formulas/mathematics/high-school/d24lg9jw8oxscaidyc9d4lybv7yybcmfuo.png)
Explanation:
Time can be calculated by dividing the distance by the speed.
![t=(d)/(s)](https://img.qammunity.org/2021/formulas/mathematics/middle-school/dr21dc8riffitfyo4az5x17f46kxna2uxs.png)
where d is the distance and s is the speed.
The distance is 90 miles and the speed is 30 miles per hour.
![d= 90 \ mi \\s= 30 \ mi/hr](https://img.qammunity.org/2021/formulas/mathematics/high-school/a0rfbzjaquhf9wv3hjjyppycb2ezby41ym.png)
![t=(90 \ mi)/(30 \ mi/hr)](https://img.qammunity.org/2021/formulas/mathematics/high-school/2p710pqgpunva7yk1nhnzyb8ig6hzontlr.png)
Divide. Note that the miles, or mi will cancel each other out.
![t=(90)/(30 \ hr)](https://img.qammunity.org/2021/formulas/mathematics/high-school/l7nw7sit89coqfvz0h1drscceg0f5st8s3.png)
![t= 3 \ hr](https://img.qammunity.org/2021/formulas/mathematics/high-school/a9n7pu9yp6pun5rj7c7rkeobrzwlw9l39c.png)
It will take 3 hours to travel 90 miles at 30 miles per hour.